The aurora borealis happens when charged particles from the Sun collide with gases high in Earth’s atmosphere, guided toward the polar regions by Earth’s magnetic field. Those collisions make the atmosphere glow, creating the shifting curtains of green, red, and purple light people call the northern lights. It is most visible on dark, clear nights, usually near the Arctic Circle, and sightings are strongest in late fall, winter, and around the spring and autumn equinoxes.

The basic cause is solar activity. The Sun constantly sends out a stream of charged particles, but bursts like solar storms and coronal mass ejections can send a much larger number of them toward Earth. When these particles reach our planet, most are deflected, but some get trapped by the magnetic field and funneled toward the north and south poles. In the upper atmosphere, they smash into oxygen and nitrogen atoms, transferring energy that is released as light. That is why the aurora is not just a single glow, but a moving display of arcs, waves, and curtains.

The colors depend on what the particles hit and how high up the collisions happen. Oxygen often produces green light, which is the most common color, while red can appear at higher altitudes. Nitrogen can add blue or purple tones. The result is a sky that can look calm and delicate one minute, then suddenly bright and active the next.

Timing matters just as much as location. The northern lights can appear throughout the year, but they are only easy to see when the sky is dark enough. In practice, that usually means late August through mid-April in many northern locations, with the darkest and most reliable viewing often from November through February. They are especially strong around March-April and September-October, near the equinoxes, when the geometry of the Sun’s and Earth’s magnetic fields tends to favor more auroral activity.

Even on a good night, the aurora is never guaranteed. Cloud cover, moonlight, and light pollution can hide it, and a quiet Sun may produce only faint displays. But when conditions line up, the experience can be unforgettable: a cold, clear sky suddenly comes alive with rippling light that seems almost to move with its own rhythm.